Process for the manufacture of oxygen-enriched air



June 28, 1966 E. CARBONELL 3,257,814

PROCESS FOR THE MANUFACTURE OF OXYGEN-ENRICHED AIR Filed Jan. 2, 1963 5 Sheets-Sheet 1 June 28, 1966 E. CARBONELL 3,257,314

PROCESS FOR THE MANUFACTURE OF OXYGEN-ENRICHED AIR 5 Sheets-Sheet 3 Filed Jan. 2, 1963 June 28, 1966 E. CARBONELL 3,257,814

PROCESS FOR THE MANUFACTURE OF OXYGEN-ENRICHED AIR Filed Jan. 2, 1963 5 Sheets-Sheet 4 Wyn 7M i/mf [Mao/viz;

June 28, 1966 E. CARBONELL 3,257,814

PROCESS FOR THE MANUFACTURE OF OXYGEN-ENRICHED AIR Filed Jan. 2, 1963 5 Sheets-Sheet 5 [MM 5 lMM/VELL United States Patent 3,257,814 PROCESS FOR THE MANUFACTURE OF OXYGEN-ENRICHED AIR Em le Carbonell, Paris, France, assignor to lAir Liqulde Societe Auonyme pour lEtude et lExploitation des Procedes Georges Claude Filed Jan. 2, 1963, Ser. No. 249,056 Claims priority, application France, Jan. 5, 1962, 883,934, Patent 1,322,843; Apr. 13, 1962, 894,365, Patent 82,408

4 Claims. (Cl. 62-29) This invention relates to a method for the production of oxygen-enriched air through low-temperature liquefaction and rectification, in which a portion at least of the air to be separated is compressed, purified and cooled close to its dew point, then subjected to a fractionate condensation under reflux, and the liquid oxygen-enriched fraction is expanded, then vaporized through indirect heat exchange with the condensing air, while the residual nitrogen gas is also expanded, then warmed up close to room temperature and discharged.

Such methods, known as back-return methods, have been known for a long time; they werefor instance the subject of French Patent No. 324,460 and of its first certificate of addition No. 1,089 of Georges Claude. They were later abandoned for separation in rectifying columns, which makes it possible to obtain pure oxygen and pure nitrogen. Even when it was wished merely to obtain oxygen-enriched air, notably for enriching furnace blasts, it was deemed cheaper to produce essentially pure oxygen through rectification, then to mix it with air, rather than to produce enriched air directly through fractionate condensation.

An object of this invention is to allow the direct production of oxygen-enriched air containing 40% to 45% oxygen by volume, without a rectifying column and with a significantly smaller expense of refrigeration energy than through the preparation of essentially pure oxygen in rectifying columns or through the use of the back return apparatus of French Patent No. 324,460. This is achieved through compressing the air to a comparatively low pressure, 2.3 to 3 absolute atmospheres, i.e. significantly less than in the known methods. The equipment is extremely simple and comparatively cheap; besides the air compressor and heat exchangers, among which one reflux exchanger, it requires only one expanding machine, preferably a turbine, in view of the low differential pressure and of the high fiow rate of the gas to be expanded.

The method of the invention is characterised in that the oxygen-enriched fraction is supercooled, before it is expanded, through heat exchange with the previously expanded residual nitrogen.

This gives an extremely cold liquid, the progressive vaporization of which, counter-current to the compressed air under fractionate condensation, allows an almost complete retention of the oxygen in the liquid fraction, andthe separation, at the cold end of the reflux exchanger, of a gas which is almost pure nitrogen.

The elimination of carbon dioxide from the air to be separated may be effected through any known means: chemically (absorption of carbon dioxide by a solution of sodium hydroxide), through freezing in an exchanger with recurrent defrosting, or through adsorption. However, the preferred method, according to the invention, is the elimination of carbon dioxide in a heat exchanger with recurrent permutation of gas streams (regenerator or reversing exchanger) through freezing of the carbon dioxide during cooling, followed by the vaporization of the latter in both separated fractions, the difference of temperatures at the cold end of the exchanger where the 3,257,814 Patented June 28, 1956 carbon dioxide deposits being kept sufiiciently low to allow the complete vaporization of the carbon dioxide by warming up an auxiliary stream of gas through countercurrent passage of the air to be cooled through an additional compartment of the said exchanger. This auxiliary stream, according to a first embodiment, is advantageously constituted by a portion of the compressed nitrogen gas from the reflux condenser, which is then combined with the other portion before it is expanded with external work.

According to another embodiment, it can also be a stream of nitrogen circulating through a closed circuit, which is warmed up in the additional compartment of the exchanger, then warmed up close to room temperature, compressed, cooled again close to its temperature at the outlet of the additional compartment, then expanded with external work and reintroduced into the cold end of the additional compartment. However, this requires the use of an additional compressor and expanding machine, as well as of another exchanger, or the adjunction of addition exchange compartments to an existing exchanger.

The auxiliary gas stream can also be a fraction of the air to be separated, bled at the cold end'of the exchanger where the carbon dioxide deposits, then expanded after warming up and combined with the nitrogen separated at low pressure at the cold end of the said exchanger.

An installation for carrying out the method of the invention essentially includes a low-pressure air compressor (2.3 to 3 absolute atmospheres); a warm exchange device in the range between room temperature and about 180 C, a cold exchange device in the range between about 180 and the dew point of the air under the relevant pressure; a reflux condensing device ensuring the fractionate condensation of the air, thesupercooling and vaporization of the oxygen-enriched liquid; and an expansion turbine. The various heat-exchange devices can be tubular exchangers of the usual type, regenerators or reversing exchangers.

More particularly, the fractionated condensation of air can be carried out by means of a reflux condenser of the type described in French Patent No. 1,123,353 for another application (condensation of ethylene from furnace gases), or in British Patents Nos. 783,186 (September 18-, 1957) and 843,119 (August 4, 1960), or in British Patent No. 847,523 (September 7, 1960), the oxygenenriched liquid being super-cooled in a special exchanger. Both operations can also be carried out at once by means of a battery of three regenerators with circular permutation, each one being successively traversed by cold air under pressure, by the oxygen-enriched liquid (previously expanded), and by nitrogen (previously expanded with external work).

Another object of the invention is to allow the production of air with a higher oxygen content than by the above method, and more particularly of superoxygenated air with an oxygen content between and by volume, more especially suitable for some applications in iron metallurgy, particularly for enriching furnace blasts. The French patent application No. P.V. 889,349 of February 27, 1962 already discloses a method for producing air with about 70% oxygen content from air compressed to 3.5-4 absolute atmospheres, thus with a notably smaller expense of energy than in air separation installations of the usual type. However, the latter method requires much the same equipment as the known methods, notably two successive rectifying columns under difincludes, beside the reflux condenser described above,

only one rectifying column. It also makes it possible, if

desired, to produce a certain amount of essentially pure oxygen through the adjunction of a small auxiliary rectifying column, working under the same pressure as the main column.

An additional object of the invention is to make it possible to produce superoxyge'nated air, and incidentally a certain amount of pure oxygen, with a reduced expense of energy.

To effect the separation of the air, one merely has to compress a portion between 2 and 3 absolute atmospheres, and another portion between 4 and 5 absolute atmospheres. The required amount of cold can be obtained through expansion with external work, either of a portion of the treated air, or of the separated nitrogen.

This embodiment is characterised in that the abovementioned oxygen-enriched vaporized fraction or part of that fraction is fed into the bottom of a rectifying column, into the top of which is fed the nitrogen separated in the reflux condensation, after liquefaction through heat exchang with liquid oxygen containing about 6070% 0 and expansion to a pressure close to the pressure of the oxygen-enriched vaporized fraction.

This embodiment of the invention also involves the following variations, taken together or separately:

(a) The gas fraction subjected to expansion with external work is the nitrogen separated at the top of the rectifying column and thereafter partly warmed up;

(b) The gas fraction subjected to expansion with external work is an air fraction compressed to a lower pressure than the air subjected to reflux condensation, and also cooled through heat exchange with the products of the separation through rectification;

(c) The air fraction expanded with external work is fed into the middle zone of the rectifying column;

(d) The oxygen-enriched liquid fraction separated in the reflux condensation through heat exchange withthe nitrogen separated at the top of the rectifying column, then expanded with external work according to (a), is indirectly supercooled: the liquid nitrogen from the reflux condensation is supercooled by means of the nitrogen gas expanded with external work, before the liquid nitrogen is expanded and introduced at the top of the rectifying column; the nitrogen gas from the head of the rectifying column is then warmed up through heat exchange with the oxygen-enriched liquid fraction;

(e) The oxygen-enriched liquid fraction separated in the reflux condensation through heat exchange with the fraction expanded with external work according to (b) is indirectly supercooled by means of the nitrogen gas from the top of the rectifying column, into which is fed the air expanded with external work;

(f) An additional fraction of, the air to be separated, under a pressure close to the pressure of the air sent to the reflux condensation, is liquefied through heat exchange withthe liquid oxygen (60% to 70% O separated at the bottom of the rectifying column, then expanded and introduced into the said column;

(g) A fraction of the liquid oxygen (about 70% 0 from the bottom of the rectifying column is separated in an auxiliary rectifying column into essentially pure liquid oxygen and a gas fraction which is returned to the main rectifying column;

(h) Another additonal air fraction, under a pressure close to the pressure of the air sent to the reflux condensation, is liquefied through heat exchange with the auxiliary rectifying column according to (-g), then expanded and fed. into the main rectifying column.

However, when an auxiliary air fraction, under a comparatively low pressure (below the pressure of the air sent to the reflux condensation), is fed in the gaseous state into the rectifying column, this involves comparatively complex air-cooling devices, with three separate heat-exchange lines, for instance in exchangers with recurrent permutation of the air and separation-products streams, as will be described and shown in relation with FIG. 5. In a first line of exchange, the air going to reflux condensation, compressed to an intermediate pressure, is cooled counter-current to part of the nitrogen separated under low pressure. In a second line, part of the air to be blown into the rectifying column, compressed to a comparatively low pressure, is cooled through heat exchange with the 60-70% oxygen separated. In a third line, another portion of the air compressed to a comparatively low pressure is cooled through heat exchange with another portion of the nitrogen separated under low pressure. Those three separate heat-exchange lines are provided because the flow rate of low-pressure nitrogen is considerably higher than the flow rate of intermediatepressure air, while the flow rate of low-pressure air is considerably higher than the flow rate'of low-pressure 60-70% oxygen.

The excess low-pressure air is therefore cooled through heat exchange with the excess low-pressure nitrogen.

Although this arrangement ensures adequate cooling and purifying of the air, it requires a large number of exchangers with recurrent inversion of the gas streams, i.e. of comparatively costly pieces of equipment, which notably increase the total cost of the installation for the production of 60-70% oxygen, compared with what might be expected from an air cooling and purifying device as simple as those in current use.

The improvement defined hereafter is meant to make it possible to carry out the cooling and purifying of the bulk of the air in regenerators less costly than exchangers with recurrent inversion of the gas streams; it makes it possible to use one such exchanger only, which treats a small flow of air. This improvement is characterised in that the auxiliary air fraction is cooled and purified counter-current to the gas containing 60-70% 0 in a first heat-exchange line made of a first battery of regenerators; the air fraction to be condensed under reflux is cooled and purified counter-current to the nitrogen separated in a second heat-exchange line made of a second battery of regenerators, while another auxiliary air fraction, purified separately from its cooling, is cooled in a third heat-exchange line, counter-current to a portion of the separated nitrogen, taken from the excess nitrogen over and above the air fraction to be condensed under reflux, in the middle zone of the second battery of regenerators, and is afterwards fed in the gaseous state into the middle zone of the rectifying column; part of the first auxiliary air fraction is bled from the middle zone of the first battery of regenerators, with such a flow rate that the residual air flow is lower than the flow of 6070'% oxygen gas in the coldest zone of the first battery of regenerators, then purified and also fed in the gaseous state into the middle zone of the rectifying column.

With reference to the appended drawings, several installations for the production of oxygen-enriched air according to the invention are described hereafter.

FIG. 1 of the drawing shows an installation in which the complete sublimation of the carbon dioxide deposited in the reversing heat exchangers is ensured through the circulation, in a special compartment of those exchangers, of a nitrogen fraction taken from the outlet of the reflux condenser, then warmed up and combined with the main nitrogen fraction under pressure at the inlet of the expansion turbine.

FIG. 2 shows an installation in which the sublimation is replaced by a set of three regenerators with circular permutation.

FIG. 5 shows an installation for the production of oxygen containing about 6070% 0 in which a rectifying column is added to the reflux condenser.

FIG. 6 shows an installation for the production of oxygen containing about 60-70% 0 similar to the installation shown on FIG. 5, but with an additional auxiliary rectifying column for the production of a small amount of 99.5% oxygen.

FIG. 7 shows an installation for the production of oxygen containing about 6070% 0 similar to the installation shown on FIG. 5, but where the air is cooled through heat exchange with the separated nitrogen and oxygen in two separate batteries of regenerators.

Referring to FIG. 1, the air to be separated is raised by turbo-compressor 10 to 2.7 absolute atmospheres. It is fed through duct 11 into reversing heat exchanger 1, where it is cooled to about 95 C., exchanging heat with the separated enriched air and nitrogen, and where the moisture is deposited as ice. It then goes through duct 14 to exchanger 2, with ternary permutation of the air, nitrogen and oxygen circuits, where it is cooled to about 180 C., and through duct 17 to exchanger 3 where it is cooled to approximately 183 C., close to its dew' point.

The cooled and purified air is then fed through duct 22 into the bottom of reflux condenser 4, where it is cooled through indirect contact counter-current to the oxygen-rich liquid separated in this condenser, supercooled and expanded; it condenses progressively while flowing upwards in the exchanger, and the liquid thus formed runs down counter-current to the ascending gas. At the bottom of this condenser, a liquid containing 40-45% 0 is collected in vessel 30; the gases from that liquid are returned through duct 31 to the bottom of condenser 4.

Owing to the counter-current exchange of matterbetween liquid and gas, there emerges from the top of condenser 4, through duct 34, essentially oxygen-free nitrogen. The first and larger fraction is sent through valve 36 to duct 51, after addition of another fraction warmed up in exchanger 3, as described below; to the nitrogen under pressure is added through duct 50 a fraction of nitrogen under pressure previously warmed up in exchangers 3 and 2, then cooled again in exchanger 3; it is then sent to valve 39, which controls its pressure before it reaches turbine 41 through duct 40; it is expanded in this turbine close to atmospheric pressure, then sent through duct 42 to exchanger 5, which is meant to supercool the oxygenenriched liquid separated in the reflux condenser. Thus warmed up to approximately -187 C., the low-pressure nitrogen. goes through duct 23 to exchanger 3, then through duct 13 to exchanger 2 and through duct to exchanger 1; it is finally discharged through duct 12 close to room temperature.

The second fraction of nitrogen under pressure is sent through duct 25, with flow regulation by valve 35, to a special compartment of exchanger 3, where it warms up countercurrent to the separated enriched air and nitrogen. It emerges from the warm end of this exchanger through duct 26; one portion is directly combined through valve 37 with the first nitrogen fraction from valve 36; another portion is sent through duct to a special compartment of exchanger 2, where it warms up again; it is then returned through duct 21 to a special tube nest of exchanger 3, where it cools down again before it is combined through duct 50 with the nitrogen fraction fed through duct 51 into turbine 41.

The oxygen-enriched liquid collected in vessel 30 is sent through duct 32 to exchanger 5, where it is supercooled to approximately -189 C. through heat exchange with the cold nitrogen expanded with .external work (see above). It is then expanded in valve 33 whose down- This nitrogen is divided into two fractions.

6 stream side is near atmospheric pressure, and returned through duct 27 into reflux condenser 4, where it is vaporised through heat exchange with air under pressure. Owing to the low temperature of the oxygen-enriched liq uid, due to its previous supercooling, an excellent separation of the constituents of the air to be condensed is obtained, so that the liquid collected at the bottom of the reflux condenser is very high in oxygen to The oxygen-enriched air emerging from the bottom of reflux condenser 4 is sent through duct 24 to exchanger 3, then through duct 19 to exchanger 2 and through duct 16 to exchanger 1; it finally emerges close to room temperature through duct 13 and is sent to the point of use.

The installation shown on FIG. 2 is largely similar to the installation shown on FIG. 1, but the difference of temperatures at the cold end of the exchangers where carbon dioxide deposits is maintained, not through warming up a fraction of the nitrogen separated under pressure, but through a closed cycle of nitrogen. A stream of nitrogen, compressed by turbo-compressor to a pressure between 5 and 15 kg./sq. cm., is sent through duct 51 to exchanger 1, where it cools down counter-current to the air to be separated, then through duct 52 to turbine 53, where it expands. Greatly cooled by that expansion, it is introduced through duct 54 into the cold end of a special compartment of exchanger 2; at the outlet of the latter, it goes through duct 55 to exchanger 1, where it warms up close to room temperature, counter-current to compressed nitrogen, before it reaches the inlet of turbocompressor 50.)

Instead of cooling the compressed nitrogen and warming up the low-pressure nitrogen returned to the compressor inside compartments of exchanger 1, this may of course be done in a separate exchanger.

The above nitrogen cycle therefore maintains inside exchanger 2 a flow of cold gas higher than the flow of the air to be cooled, since the nitrogen expanded in turbine 53 has not been previously cooled in that exchanger. As to exchanger 3, the excess flow of cold gas is maintained as in the installation shown in FIG. 1, through warming up in a special compartment of that exchanger a fraction of the nitrogen under pressure separated in the reflux condenser.

The installation shown in FIG. 3 is also similar to the installation shown in FIG. 1, but the excess flow of cold gas inside exchanger 2 is maintained through bleeding at the cold end of that exchanger 21 small fraction of the cooled air under pressure, which is introduced through expanding valve into a special compartment of that exchanger; after having warmed up, this air fraction is combined, through duct 61 and valve 62, with the low-pressure nitrogen introduced into the cold end of that exchanger through duct 18; part of it may also be diverted and combined, through duct 63 and valve 64, with the low-pressure nitrogen introduced through duct 23 into the cold end of exchanger 3.

Although the above installations involve the use of exchangers with recurrent inversion of the air and nitrogen circuits, the latter may be replaced by regenerators, provided with internal tube nests to carry the nitrogen under pressure to be expanded with external work, which must not be polluted by the carbon dioxide deposited from the air. More particularly, as shown in FIG. 4, the reflux condenser :may be replaced by a set of three regenerators, successively traversed by the air to be condensed, by the nitrogen expanded with external work, and by the oxygenenriched liquid, counter-current to the above air.

FIG. 4 shows only the three regenerators 4A, 4B and 4C, which replace the reflux condenser and ensure the supercooling of the oxygen-enriched liquid.

In the stage shown in FIG. 4a, the air close to its dew point is fed through duct 22 into the bottom of regenerator 4A, previously cooled by nitrogen expandedwith external work, where it is partly condensed under reflux as it rises through the packing. There emerges from the bottom of the regenerator, through duct 32, an oxygenenriched liquid, which is expanded in valve 33 and warmed up in regenerator 4C, then discharged through duct 24.

The essentially pure nitrogen separated at the top of regenerator 4A through duct 34 is warmed up, expanded with external work in a turbine (not shown), then warmed up by passing through regenerator 4B and discharged through duct 23.

When regenerator 4A has warmed up to such an extent that the separation of the air becomes inadequate, the regenerators undergo a circular permutation: air is introduced into regenerator 4B, while regenerator 4A is cooled by the expanded oxygen-enriched liquid, and regenerator 4C by the nitrogen expanded with external work (FIG. 4b).

Finally, in a third stage, air at the dew point is introduced into regenerator 4C, while regenerators 4A and 4B are cooled respectively by low-pressure nitrogen and by oxygen-enriched liquid.

In the installation shown in FIG. 5, the air to be separated is introduced through duct 11 into turbo-compressor 12, which discharges it under 2.4 absolute atmospheres into duct 13. It is then divided into two fractions. The first fraction, which makes up about 67% of the whole, is compressed again by turbo-compressor 15 to 4.3 absolute atmospheres, then through duct 16, counter-current to a portion of the nitrogen separated under low pressure, into exchangers 1C and 2C, with recurrent inversion of the air and nitrogen streams by means of two sets of valves (21A, 21B, 22A, 22B, and 71A, 71B, 72A, 723). These exchangers can be for instance compact weldedalu-minum exchangers of the type sold by Compagnie Europenne de Matriels Thermiques. The air cools down in these exchangers, and deposits its moisture and carbon dioxide in the solid state. To ensure complete sublimation into the separated nitrogen of the carbon dioxide previously deposited, a portion (about 18%) of the cooled air is bled through duct 80, with flow control by valve 8-1, and returned counter-current to the air to be cooled into a special compartment of the exchanger, so as to reduce the difference of temperatures between air and nitrogen at the cold end of the exchanger. The main portion of the air is combined through valve 82 with the warmed-up portion, and the combined streams are fed through duct 83 into exchanger 3, where they are cooled down close to the dew point through heat exchange with the nitrogen separated in rectifying column 6, and with part of the nitrogen from expansion turbine 7, as described below.

The first air fraction under higher pressure, close to its dew point, is fed through duct 84 into the bottom of reflux condenser 4, where it rises with progressive condensation through indirect contact counter-current to the oxygen-enriched liquid separated in the same condenser, then supercooled and expanded; there emerges at the bottom of the said condenser, through duct 32, a liquid containing to 0 which is collected in vessel 33; the gases from that liquid are returned through duct 34 to the condenser.

The oxygen-enriched liquid from vessel 33 is sent through duct 35 to exchanger 5, where it is supercooled countercurrent to pure nitrogen from the rectifying column, as described below; it then goes through duct 36 to expanding valve 37 (2.4 atmospheres) and is returned to reflux condenser 4, where it is vaporized through indirect heat exchange with the air under pressure being condensed. It is then fed through duct 38 into the bottom of rectifying column 6.

The nitrogen emerging from the top of reflux condenser still contains about 1% to 5% oxygen. This nitrogen goes through duct 39 to exchanger 40, where it is condensed through heat exchange with superoxygenated air (-70% 0 separated in rectifying column 6, as described below. It then goes through duct 41 to exchanger 42, where it is supercooled through heat exchange with nitrogen expanded in turbine 7, and is fed through duct 43 into the top of rectifying column 6, as reflux liquid, after having been expanded to 2.4 atmospheres in valves 44.

The second air fraction, under 2.4 atmospheres, is divided in two portions at the outlet of duct 14. The first portion is cooled and purified, counter-current to oxygen, in reversing exchangers 1A and 2A, fitted with valve sets 17A, 17B, 18A, 18B and 63A, 63B, 64A, 6413, which ensure the recurrent inversions between air and oxygen. As described above for exchanger 2C, complete sublimation of the deposited carbon dioxide is ensured by returning a portion of the cooled air, bled through duct 73 and warmed up counter-current to the incoming air, with flow control by valve 74. The air thus warmed up is then mixed again with the main fraction going through valve 75, and the combined streams go through duct A.

The second portion of the low pressure air is cooled and purified counter-current to a portion of the lowpressure nitrogen in reversing exchanger 18, fitted with two sets of valves 19A, 19B, 20A, 20B and 67A, 67B, 63A, 68B. Exchanger 2B also contains a tube nest for the return of the air bled through duct 76 with flow control by valve 77. After this air has been combined by valve 78 to the main stream of air, the second portion of cooled low-pressure air goes through duct 79.

Complete sublimation of the deposited carbon dioxide may of course be obtained by means of a closed circuit of nitrogen, or of air bleeding warmed up in special tube nests, then combined with separated nitrogen or oxygen, according to the variations shown in FIGS. 2 and 3.

Both portions of cooled low-pressure air are then combined in duct 31 and fed in the gaseous state into the middle zone of rectifying column 6.

The low-pressure air and the oxygen-enriched gas fed in the gaseous state into that column through duct 38 are separated therein into a liquid containing about 60% to 70% oxygen, essentially in equilibrium with the gas fed in the gaseous state through duct 38, and into essentially pure nitrogen. The superoxygenated liquid air is sent through duct 45 and expanding valve 46, under approximately 1.2 atmospheres, to exchanger 40, where it is vaporized through heat exhange with the nitrogen issuing from reflux condenser 4; it then goes through ducts 47 and 62 to heat exchangers 2A and 1A, where it warms up close to room temperature, before it is sent to use by duct 90.

The essentially pure nitrogen emerging from the head of column 6 is sent through duct 48 to exchange 5, where it warms up through heat exchange with the oxygen-enriched liquid separated in reflux condenser 4, then through duct 49 to exchanger 3, where it warms up again, and is then fed through duct 50 into expansion turbine 7 under approximately 1.2 atmospheres. It then ensures in exchanger 42 the supercooling of the liquid nitrogen sent to column 6, then goes through ducts 51, 52 and 53 respectively to heat exchangers 3 and 2C, 1C or 2B, 1B, before it is discharged through ducts 26 or 28, then 29.

The installation for the separation of air shown in FIG. 6, which allows the production of some pure oxygen (about 10% of the separated oxygen), contains many elements which are essentially similar to those in the installation shown in FIG. 5, particularly air compressors 12 and 13, heat exchange zones 1 and 2, shown each as a single exchanger for simplicity, and reflux condenser 4, the operation of which shall not be reiterated. The pressures of both air fractions are slightly higher than those of FIG. 5: 2.5 to 3 absolute atmospheres for the low-pressure fraction and 4.5 to 5 absolute atmospheres for the high-pressure fraction. Cold is produced by expanding the low-pressure air fraction with external work in turbine 7 before it is fed into the main rectifying column 6, which works under approximately 1.2 atmospheres.

On the other hand, a portion only of the high-pressure air fraction is fed into reflux condenser 4, as above. Another portion is sent through duct 60 to exchanger 61, where it condenses through heat exchange with 60-70% liquid oxygen, bled from the bottom of main column 6 through duct 45 and already partly vaporized in exchanger 40. It then goes through duct 62 to expanding valve 63 (1.2 atmospheres), and is then fed through duct 64 into column 6. A third portion is sent through duct 65 to coil 66, laid inside the sump of auxiliary rectifying column 8, where it condenses by warming up the bottom of that column. It then goes through duct 67 to expanding valve 68 (1.2 atmospheres), and is then fed, also through duct 64, into column 6.

The supercooling of liquid nitrogen in condenser 42 is not effected by nitrogen expanded with external work, but 'by cold nitrogen issuing from the head of column 6 through duct 48.

On the other hand, a small fraction of the liquid containing 6070% and issuing from the bottom of the main rectifying column 6 is sent through duct 60 to the top of an auxiliary column 8, which separates it into I essentially pure liquid oxygen and oxygen-poor gas, the

latter being returned through duct 70 to the bottom of.

column 6. The essentially pure oxygen is bled from column 8 in the gaseous state through duct 71, then warmed up in heat-exchange areas 2 and 1, co-current to superoxygenated air (6070% 0 and finally sent to use, close to room temperature, through duct 73.

Other variations of the present improvement may of course be carried out without diverging from its principle. For instance, a small amount of pure oxygen can be obtained by operating both rectifying columns under higher pressures and effecting refrigeration through expanding with external work the essentially pure nitrogen separated.

1n the installation of FIG. 7, the low-pressure air introduced through duct 14, is divided into a first auxiliary fraction going through duct 114 and another auxiliary fraction, with a much lower flow rate, going through duct 214. The first auxiliary fraction is cooled and purified in regenerators 191A and 191B, through which the streams of air and of 6070% oxygen are circulated alternatively, by means of a set of valves 192A, 192B, 193A, 193B, and of the valve boxes shown schematically in 194A, 194B. In the working stage shown, valves 192A and 19313 are open, valves 1923 and 193A are closed, and the valve boxes have the arrangement shown, so that the air goes through regenerator 191A and coo-ls down through contact with the latters packing, while depositing its carbon dioxide in the solid state in the coldest zone of the regenerator; on the other hand, the oxygen at 60-70% 0 warms up through contact with the packing of regenerator 1918. The cooled and purified air then goes through ducts 115 and 75A to exchanger 3, then through duct 31 to the rectifying column.

To ensure the presence in the coldest zone of the regenerators of an excess flow of cold oxygen, so that the latter may ensure complete vaporization of the deposited Dry Ice, part of the air being cooled is bled through valve 195A, which opens into the middle zone of regenerator 191A (average temperature: approximately 120 C.), and is combined through duct 196 (the corresponding valve 195B, linked to regenerator 191B, being closed) to the other auxiliary air fraction, previously cooled in reversing exchanger 1A, at the inlet of adsorbing body 104, which may be made of silica gel, and which ensures the purification of the air going through it; the stream is then combined through duct 215 with the air issuing through duct 115 from the cold ends of regenerators 191A and 191Bv Although one absorbing body 104 only is shown, for the sake of clearness, two such bodies are usually provided in parallel, one in the working stage and the other in the regeneration stage.

The other auxiliary air fraction, which has a comparatively low flow rate, so computed as to allow the recovery of cold from the low-pressure nitrogen bled from the middle zone of regenerators 91A and 91B, goes through duct 214 to reversing exchanger 1A, which is provided with two sets of reversing valves A, 100B, 101A, 101B and 102A, 102B, 103A, 103B. In the working stage shown, valves 100A, 101B, 103A and 102B are open, and the other valves are closed. The air is fed into the exchanger through valve 100A and leaves it through valve 103A, while the nitrogen issuing from the middle zone of regenerator 91B through valve 95B and duct 96 is fed through valve 102B into exchanger 1A and leaves it through valve 101B; the warmed-up nitrogen is combined through duct 98 with the nitrogen issuing from the warm end of regenerator 913. The air cooled down to approximately l00 C. in the exchanger is combined, at the inlet of adsorbing body 104, with the air from the middle zones of regenerators A and 195B; the combined air streams are decarbonated, then go through ducts 75A to exchanger 3 and through duct 31 to rectifying column 6. 1

What I claim is:

1. A method for the production of oxygen-enriched air through low-temperature liquefaction and rectification, comprising the steps of:

(a) comprising and cooling an air stream close to its dew point,

(b) subjecting said air stream to a fractionate condensation under reflux by indirect heat exchange with relatively cold fluid, whereby an oxygen-enriched liquid fraction and residual nitrogen gas are obtained,

(c) liquefying said residual nitrogen gas by heat exchange with a liquid of about 60% to 70% oxygen content expanding it and feeding it into the top of a rectifying column,

(d) subcooling said oxygen-enriched liquid fraction by heat exchange with a low-pressure nitrogen stream from the top of said rectifying column, expanding it and vaporizing it by indirect heat exchange with the air stream being fractionally condensed,

(e) expanding with external work said low-pressure nitrogen stream after said subcooling step and recovering its refrigeration,

(f) feeding said vaporized oxygen-enriched fraction into the bottom of said rectifying column, and

(g) separating it within said column into said liquid of about 60% to 70% oxygen and a nitrogen stream.

2. A method according to claim 1, wherein said lowpressure nitrogen stream, after said expansion with external work, is used to subcool said liquefied residual nitrogen gas prior to expansion of said liquefied residual nitrogen gas.

3. A method according to claim 1, wherein an auxiliary air stream is compressed to a lower pressure than said air stream, then blown into the middle zone of said rectifying column.

4. A method according to claim 1, wherein said auxiliary air stream is cooled and purified countercurrent to the gas of about 60% to 70% oxygen obtained after the liquefaction of said residual nitrogen gas in a first heat-exchange line made of a first set of regenerators, said air stream is cooled and purified countercurrent to said separated nitrogen stream in a second heat-exchange line made of a second set of regenerators, while a second auxiliary air stream, purified independently from its cooling, is cooled in a third heat-exchange line countercurrent to a portion of the separated nitrogen taken from the excess nitrogen separated over and above said air stream and bled from the middle zone of said second set of regenerators, then said second auxiliary air stream is also fed in the gaseous state into the middle zone of said rectifying column, and part of the first auxiliary air stream is bled from the middle zone of said first set of regenerators with such a flow rate that the residual flow of air is lower than the flow of gas of about 60% to 70% oxygen in the coldest zone of said first set of regenerators and is then purified and also fed in the gaseous state into the middle zone of said rectifying column.

References Cited by the Examiner UNITED STATES PATENTS Gobert 6229 Schlitt 6230 X Schilling 6214 Cooper 6214 Rice 6214 X Scharmann 6213 Johnson 6214 Jakob 6214 X Shaperdas 6214 Jakob 62-14 X Grunberg 6229 X Becker 6213 Lehmer 6212 FOREIGN PATENTS Great Britain.

NORMAN YUDKOFF, Primary Examiner. 

1. A METHOD FOR THE PRODUCTION OF OXYGEN-ENRICHED AIR THROUGH LOW-TEMPERATURE LIQUEFACTION AND RECTIFICATION, COMPRISING THE STEPS OF: (A) COMPRISING AND COOLING AN AIR STREAM CLOSE TO ITS DEW POINT, (B) SUBJECTING SAID AIR STREAM TO A FRACTIONATE CONDENSATION UNDER REFLUX BY INDIRECT HEAT EXCHANGE WITH RELATIVELY COLD FLUID, WHEREBY AN OXYGEN-ENRICHED LIQUID FRACTION AND RESIDUAL NITROGEN GAS ARE OBTAINED, (C) LIQUEFYING SAID RESIDUAL NITROGEN GAS BY HEAT EXCHANGE WITH A LIQUID OF ABOUT 60% TO 70% OXYGEN CONTENT EXPANDING IT AND FEEDING IT INTO THE TOP OF A RECTIFYING COLUMN, (D) SUBCOOLING SAID OXYGEN-ENRICHED LIQUID FRACTION BY HEAT EXCHANGE WITH A LOW-PRESSURE NITROGEN STREAM FROM THE TOP OF SAID RECTIFYING COLUMN, EXPANDING IT AND VAPORIZING IT BY INDIRECT HEAT EXCHANGE WITH THE AIR STREAM BEING FRACTIONALLY CONDENSED, (E) EXPANDING WITH EXTERNAL WORK SAID LOW-PRESSURE NITROGEN STREAM AFTER SAID SUBCOOLING STEP AND RECOVERING ITS REFRIGERATION, (F) FEEDING SAID VAPORIZED OXYGEN-ENRICHED FRACTION INTO THE BOTTOM OF SAID RECTIFYING COLUMN, AND (G) SEPARATING IT WITHIN SAID COLUMN INTO SAID LIQUID OF ABOUT 60% TO 70% OXYGEN AND A NITROGEN STREAM. 